1. Field of the Invention
The present invention relates generally to microminiature electronic elements and particularly to an improved package and method of packaging for microminiature electronic components.
2. Description of Related Technology
For many years, electronic circuit boards have been fabricated by interconnecting a plurality of electronic components, both active and passive, on a planar printed circuit board. Typically, this printed circuit board has comprised an Epoxy/fiberglass laminate substrate clad with a sheet of copper, which has been etched to delineate the conduct paths. Holes were drilled through terminal portions of the conductive paths for receiving electronic component leads which were subsequently soldered thereto.
More recently, so-called surface mount technology has evolved to permit more efficient automatic mass production of circuit boards with higher component densities. With this approach, certain packaged components are automatically placed at preselected locations on top of a printed circuit board so that their leads are registered with, and lie on top of, corresponding solder paths. The printed circuit board is then processed by exposure to infrared, convection oven or vapor phase soldering techniques to re-flow the solder and, thereby, establish a permanent electrical connection between the leads and their corresponding conductive paths on the printed circuit board.
Dual in-line chip carriers are a well known embodiment of microelectronic component packages which have existed for many years. The most common example is an integrated circuit which is bonded to a ceramic carrier and electrically connected to a lead frame providing opposite rows of parallel electrical leads. The integrated circuit and ceramic carrier are normally encased in a rectangular plastic housing from which the leads extend. Typically, these dual in-line packages are mounted horizontally, i.e. with the leads extending co-planar with the printed circuit board. Such dual in-line packages have heretofore been attached to printed circuit boards by surface mounting techniques.
The increasing miniaturization of electrical and electronic elements and the high density mounting of such elements has created increasing problems with electrical isolation and mechanical interconnection. In particular, miniaturization and high density placement makes it more difficult to establish reliable and efficient connection between fine gauge (AWG 20 to AWG 50) copper wires and egress hardware or terminals. Presently known interconnection methods severely limit the ability to provide high density and reliable electrical and mechanical isolation between distinct egress or terminal points due to space limitations.
One prior art interconnection approach, as illustrated in FIG. 1, is to extend a fine copper wire forming the element lead and to wrap or coil it around a terminal pin of a terminal and apply solder to the connection. This configuration requires space that is not always available and does not allow adequate separation for high voltages that may be required in the circuit. Another problem with this approach is that element leads are frequently broken or sheared during a subsequent encapsulation process. In addition, the lead is also frequently broken as the result of thermal expansion and contraction of the leads and/or terminals. For reasons discussed further below, this method is particularly unsuitable when microelectronic transformers are used within the component package. Transformers are electrical components which are used to transfer energy from one alternating current (AC) circuit to another by magnetic coupling. Generally, transformers are formed by winding one or more wires around a ferrous core. A first wire acts as a primary winding and conductively couples energy to and from a first circuit. A second wire, also wound around the core so as to be magnetically coupled with the first wire, acts as a secondary winding and conductively couples energy to and from a second circuit. AC energy applied to the primary windings causes AC energy in the secondary windings and vice versa. A transformer may be used to transform between voltage magnitudes or current magnitudes, to create a phase shift, and to transform between impedance levels.
Another purpose for which microelectronic transformers may be used is to provide physical isolation between two circuits. For example, a transformer may be used to provide isolation between a telephone signal line in the public switched telephone network and consumer equipment such as modems, computers and telephones. The transformer must be able to withstand large voltage spikes which may occur due to lightning strikes, malfuctioning equipment, and other real-world conditions without causing a risk of electrical fire or other hazardous conditions.
One well-known configuration for a microelectronic transformer comprises a toroidal ferrous core. A toroidal transformer can elegantly provide any one of the above listed functions. One drawback to the use of toroidal cores relates to manufacturing; such cores are not easily manufactured nor are the resulting transformers in a convenient configuration for modern component package production techniques. Additional information about electronic microminiature packaging can be found in U.S. Pat. No. 5,015,981 entitled "ELECTRONIC MICROMINIATURE PACKAGING AND METHOD" which is assigned to the assignee hereof, and incorporated by reference herein in its entirety.
In addition to physical and manufacturing considerations, the electrical performance of the transformer must be considered. One means by which the electrical performance of transformers is gauged is the high-potential (hi-pot) test. A hi-pot test involves the application of AC or DC signals to the transformer to determine whether the breakdown of the core dielectric or other destructive failures occur at some chosen voltage level. A hi-pot test can also be used to demonstrate that insulation can withstand a given over-voltage condition and to detect weak spots in the insulation that could later result in in-service failures.
The International Electro-Technical Committee is an international standards body which develops the standards by which isolation transformers are categorized according to performance level. For example, UL-1950 and its corresponding national adaptations specify a minimum standard for dielectric breakdown between the primary and secondary windings of a transformer. In order to meet such a standard, it is critical that the primary and secondary windings are electrically isolated from one another while remaining magnetically coupled to one another through the transformer core.
In order to provide such electrical isolation between the primary and secondary windings, special jacketing materials have been developed to encase one or both of the primary and secondary windings. For example, UL-1950 specifies that one winding is covered with three layers of insulation material for which all combinations of two layers together pass a specified electric strength test. The second winding may be enameled copper magnet wire. The wire covered in a protective extrusion or jacket comprising three layers of insulation is referred to as reinforced insulated or jacketed wire. The protective jacket provides electrical isolation which inhibits dielectric breakdown between the windings at extended voltage ranges. One example of jacketed wiring is that manufactured by the Rubadue Wire Co. of Fontana, Calif. and incorporated in Pulse Engineering, Inc. part number 054-XIXWXXX-X.
The protective jacket provides insulation between the wires so long as it is in place. But in order to conductively connect the insulated conductors to external elements during the manufacturing process, a portion of the conductors must be exposed by removal of the protective jacket. The exposed conductors are not immune from dielectric breakdown and other phenomenon which may decrease the isolation performance of the resulting transformer. Isolation between the exposed conductors and the remainder of the transformer such as the ferrous core and the second winding must be accomplished by some other means. UL-1950 specifies that the additional isolation is accomplished by physical separation between the exposed conductors and other transformer elements. For example, UL-1950 specifies that the exposed portion of the conductors should be separated from the bare core and the second windings by at least 0.4 millimeters (mm) or approximately 0.016 in. (16 mils).
One difficulty related to the incorporation of jacketed insulated wire into a dual in-line surface mount package is maintaining the 0.4 mm clearance in a repeatable and consistent manner. The jacketed wire resists bending and may tend to spring back and, thus, move after placement. In the manufacturing environment, the assembly of packaged electronic elements is carried out in stages. In order for manufacturing to produce conforming parts, it is important that the previously assembled components remain properly in place while waiting for and during execution of additional manufacturing steps. In this case, if the wires move from the desired position, the parts may not conform to the requirements (such as the aforementioned requirement of 16 mils of spacing).
Based on the foregoing, it would be desirable to provide an improved microelectronic component surface mount package and method of manufacturing. Such an improved package would provide a guaranteed separation between the exposed conductors and other component elements and also provide a locking feature to hold the jacketed wire in place during the manufacturing process, thereby allowing for a more uniform and reliable product.